HFP: Check for service level connection when connecting SCO

* A service level connection (SLC) should be established before a SCO
  connection could be created
* This CL creates a flag within HeadsetStateMachine to track the status
  of service level connections
* BluetoothHeadset.connectAudio() will return false when an SLC is not
  established

Bug: 36100309
Test: make, HFP regression, testplans/82424
Change-Id: Ibb52674ef6be75eeac66eb228e8b75d9bba1ddc6